如何初始化文件類型變量以避免“LateInitializationError”錯誤？

Question

每當我嘗試將現有用戶登錄到我的程序時，我總是收到錯誤，但我可以創建新用戶，這些新用戶在創建時自動登錄。 我已將其縮小到包含“late File _userImageFile;”的行多變的。 不確定如何正確初始化它。 我嘗試的其他事情是給它一個 null 檢查“。” 並使變量可為空。

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:chat_app2/widgets/pickers/user_image_picker.dart';

class AuthForm extends StatefulWidget {
  AuthForm(this.submitFn, this.isLoading);

  final bool isLoading;
  final void Function(String email, String password, String username,
      File image, bool isLogin, BuildContext ctx) submitFn;

  @override
  _AuthFormState createState() => _AuthFormState();
}

class _AuthFormState extends State<AuthForm> {
  final _formKey = GlobalKey<FormState>();
  var _isLogin = true;
  String _userEmail = '';
  String _userName = '';
  String _userPassword = '';
  late File _userImageFile;

  void _pickedImage(File image) {
    _userImageFile = image;
  }

  void _trySubmit() {
    final isValid = _formKey.currentState!.validate();
    FocusScope.of(context).unfocus();

    if (_userImageFile == null && !_isLogin) {
      final snackBar = SnackBar(
        content: Text("please pick an image"),
        backgroundColor: Theme.of(context).errorColor,
      );
      ScaffoldMessenger.of(context).showSnackBar(snackBar);
      return;
    }

    if (isValid) {
      _formKey.currentState!.save();
      widget.submitFn(
        _userEmail.trim(),
        _userName.trim(),
        _userPassword.trim(),
        _userImageFile,
        _isLogin,
        context,
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Card(
        margin: EdgeInsets.all(20),
        child: SingleChildScrollView(
          child: Padding(
            padding: EdgeInsets.all(16),
            child: Form(
              key: _formKey,
              child: Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  if (!_isLogin) UserImagePicker(_pickedImage),
                  TextFormField(
                    key: ValueKey('email'),
                    validator: (value) {
                      if (value!.isEmpty || !value.contains('@')) {
                        return 'Please enter a valid email address';
                      }
                      return null;
                    },
                    keyboardType: TextInputType.emailAddress,
                    decoration: InputDecoration(
                      labelText: 'Email address',
                    ),
                    onSaved: (value) {
                      _userEmail = value!;
                    },
                  ),
                  if (!_isLogin)
                    TextFormField(
                      key: ValueKey('userName'),
                      validator: (value) {
                        if (value!.isEmpty || value.length < 4) {
                          return 'Please enter at least four characters';
                        }
                        return null;
                      },
                      decoration: InputDecoration(labelText: 'Username'),
                      onSaved: (value) {
                        _userName = value!;
                      },
                    ),
                  TextFormField(
                    key: ValueKey('password'),
                    validator: (value) {
                      if (value!.isEmpty || value.length < 7) {
                        return 'Password must be at least seven characters long';
                      }
                      return null;
                    },
                    decoration: InputDecoration(labelText: 'Password'),
                    obscureText: true,
                    onSaved: (value) {
                      _userPassword = value!;
                    },
                  ),
                  SizedBox(
                    height: 12,
                  ),
                  if (widget.isLoading) CircularProgressIndicator(),
                  if (!widget.isLoading)
                    ElevatedButton(
                      child: Text(_isLogin ? 'Login' : 'Sign up'),
                      onPressed: _trySubmit,
                      style: ElevatedButton.styleFrom(
                          shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(20),
                      )),
                    ),
                  if (!widget.isLoading)
                    TextButton(
                      child: Text(_isLogin
                          ? 'Create new account'
                          : 'I already have an account'),
                      onPressed: () {
                        setState(() {
                          _isLogin = !_isLogin;
                        });
                      },
                      style: TextButton.styleFrom(
                        foregroundColor: Theme.of(context).primaryColor,
                      ),
                    ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Answer 1

當您已經在進行 null 檢查時，請使變量可為空。

File? _userImageFile;

片段

class _AuthFormState extends State<AuthForm> {
  final _formKey = GlobalKey<FormState>();
  var _isLogin = true;
  String _userEmail = '';
  String _userName = '';
  String _userPassword = '';
  File? _userImageFile;

  void _pickedImage(File image) {
    _userImageFile = image;
  }

  void _trySubmit() {
    final isValid = _formKey.currentState?.validate() ?? false;
    FocusScope.of(context).unfocus();

    if (_userImageFile == null && !_isLogin) {
      final snackBar = SnackBar(
        content: Text("please pick an image"),
        backgroundColor: Theme.of(context).errorColor,
      );
      ScaffoldMessenger.of(context).showSnackBar(snackBar);
      return;
    }

    if (isValid && _userImageFile != null) {
      _formKey.currentState?.save();
      widget.submitFn(
        _userEmail.trim(),
        _userName.trim(),
        _userPassword.trim(),
        _userImageFile!,
        _isLogin,
        context,
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Card(
        margin: EdgeInsets.all(20),
        child: SingleChildScrollView(
          child: Padding(
            padding: EdgeInsets.all(16),
            child: Form(
              key: _formKey,
              child: Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  if (!_isLogin) UserImagePicker(_pickedImage),
                  TextFormField(
                    key: ValueKey('email'),
                    validator: (value) {
                      if (value!.isEmpty || !value.contains('@')) {
                        return 'Please enter a valid email address';
                      }
                      return null;
                    },
                    keyboardType: TextInputType.emailAddress,
                    decoration: InputDecoration(
                      labelText: 'Email address',
                    ),
                    onSaved: (value) {
                      _userEmail = value??"";
                    },
                  ),
                  if (!_isLogin)
                    TextFormField(
                      key: ValueKey('userName'),
                      validator: (value) {
                        if (value!.isEmpty || value.length < 4) {
                          return 'Please enter at least four characters';
                        }
                        return null;
                      },
                      decoration: InputDecoration(labelText: 'Username'),
                      onSaved: (value) {
                        _userName = value??"";
                      },
                    ),
                  TextFormField(
                    key: ValueKey('password'),
                    validator: (value) {
                      if (value!.isEmpty || value.length < 7) {
                        return 'Password must be at least seven characters long';
                      }
                      return null;
                    },
                    decoration: InputDecoration(labelText: 'Password'),
                    obscureText: true,
                    onSaved: (value) {
                      _userPassword = value??"";
                    },
                  ),
                  SizedBox(
                    height: 12,
                  ),
                  if (widget.isLoading) CircularProgressIndicator(),
                  if (!widget.isLoading)
                    ElevatedButton(
                      child: Text(_isLogin ? 'Login' : 'Sign up'),
                      onPressed: _trySubmit,
                      style: ElevatedButton.styleFrom(
                          shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(20),
                      )),
                    ),
                  if (!widget.isLoading)
                    TextButton(
                      child: Text(_isLogin
                          ? 'Create new account'
                          : 'I already have an account'),
                      onPressed: () {
                        setState(() {
                          _isLogin = !_isLogin;
                        });
                      },
                      style: TextButton.styleFrom(
                        foregroundColor: Theme.of(context).primaryColor,
                      ),
                    ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}